The present invention relates to a method of removing alkali metal compounds from heated gaseous mixtures useful in power generation by introducing finely divided refractory oxides into the gaseous mixtures. Removal of the alkali metal compounds from the gaseous mixtures is principally achieved by the adsorption of the alkali metal compounds onto the surface of the introduced finely divided refractory oxides without effecting a reduction in the temperature of the heated gaseous mixtures.
Prior to the present invention, pressurized fluidized bed coal combustors have been evaluated for power generation in combined cycle power plants. The resulting combustion gases are cleaned of fly ash and expanded in a gas turbine generating power. The cleaned combustion gases contain high concentrations of alkali salts. Experience with oil-fired turbines has shown that gas turbine components are often severely corroded by alkali metal sulfates arising from the reaction of sulfur oxides and alkali metal salt vapors present in combustion gases.
One method of protecting the gas turbine components from alkali metal sulfate corrosion is by using a metallic coating on the surface of gas turbine components. A number of coatings based on MCrAlY compositions, where M stands for Ni, Co or Fe, or aluminide coatings having been used. However, due to severity of the gas environment in coal fired systems, alternate systems may be needed to insure adequate protection.
Another procedure which has been taught to protect gas turbine components from alkali metal corrosion is by effecting the removal of the alkali metal contaminant by condensation of the alkali compound vapor at a temperature of about 1650.degree. F. at a pressure of about 10 atmospheres onto cooler metal oxide particles at a temperature of about 1550.degree. F. or below, having a preferred diameter of from about 50 to about 150 microns introduced into the hot gaseous mixture prior to the power generation step as shown in copending application Ser. No. 17,613. Condensation of volatilized metal compounds from a gaseous stream in molten salt processing onto particulate is also shown in U.S. Pat. No. 4,120,668. Although condensation of alkali metal vapor is an effective way to remove such contaminants from hot gaseous mixtures useful in power generation, the overall efficiency of the power generation system is reduced, since the temperature of the mixture is lowered upon contacting the cooler refractory oxide particles. In addition, the alkali metal compound content of the hot gaseous mixture can be reduced only to the saturation vapor pressure which is the level above which condensation occurs. In some instances, the condensation level can be very high for particular gas compositions which may exceed the tolerance limit of the gas turbine components. Accordingly, the resulting super heated gaseous mixture can exceed the acceptable tolerence limit of 0.02 ppm which has been found suitable in oil fired turbines. It is also known that some removal of alkali metal contaminants can occur through chemical reaction with metal oxide particles such as alumino silicates. However, removal of such contaminants to acceptable levels through chemical reaction with refractory oxide particles can only occur at temperatures below gas turbine requirements.
It has been further reported by Lee and Johnson in "Removal of Gaseous Alkali Metal Compounds from Hot flue Gas by Particulate Sorbents", ASME paper No. 79-GT-154, Gas Turbine Conference, San Diego, Calif. Mar. 25-15, 1979, that alkali content of simulated flue gas can be reduced by adsorption on contact with activated bauxite at elevated temperatures.
It is an object of the present invention, therefore, to effect removal of alkali metal contamination in super heated gaseous mixtures used in power generation without effecting a substantial reduction in the temperature of the gaseous mixture.
It is a further object of the present invention to effect the removal of alkali metal contaminants from heated gaseous mixtures by introducing finely divided refractory oxide material into the super heated gaseous mixture before such heated gaseous mixture becomes in contact with gas turbine components.
Other objects of the present invention will be apparent from the following description of the present invention.